DynamoDB 推出了依照時間自動刪除的功能：「New – Manage DynamoDB Items Using Time to Live (TTL)」。
You can enable this feature on a table-by-table basis, specifying an item attribute that contains the expiration time for the item.
這個功能比較特別的是，刪除的 scan 是不收取費用的：
There is no charge for the internal scan operation or for the deletion. You will pay for storage until the item is actually deleted.